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I just came across the doubt about how the space between the stator and the rotor in Brushless DC motors affect their performance.

Of course the further apart coils and magnets are, the least torque it provides, but, smaller gaps do really mean more torque? Is there a point where some other things begin to counteract the motor performance? (not counting the obvious case where the stator literally touch the rotor)

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  • \$\begingroup\$ Welcome to EE.SE. \$\endgroup\$ – user105652 Jul 29 at 4:12
  • \$\begingroup\$ Thanks! — It's a great community <3 \$\endgroup\$ – Rhodexa Jul 29 at 6:45
  • \$\begingroup\$ Smaller gaps mean more torque and correspondingly less speed. \$\endgroup\$ – Brian Drummond Jul 29 at 13:42
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In any given motor, a higher field means better efficiency, with a lower current needed for any given torque, so lower copper losses. It also means better power per volume/cost/weight of motor.

The permanent magnets have to set up that field across the airgap. With a longer airgap, you need stronger (aka more expensive) or longer (aka heavier, more expensive and bulkier) magnets than with a motor that maintains a shorter airgap.

To maintain a small airgap without clashes between poles requires a more accurately made (more expensive) and stiffer motor.

There is a small penalty in very tight airgaps that the mechanical loss due to air viscosity increases, but that's generally outweighed by the reduction in electrical losses.

The design of any given motor is therefore a compromise between several different expensive parameters

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  • \$\begingroup\$ Oh, i get it. What an interesting topic! — Thanks for your replies \$\endgroup\$ – Rhodexa Jul 29 at 6:48
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    \$\begingroup\$ The effect on efficiency is a second order one, unless operating close to stalling. Smaller gap means higher torque ... but correspondingly less speed, for a given voltage and current. The increased current with a larger gap does increase copper losses ... but only by the wasted voltage V = IR, which is only a small percentage of the operating voltage (the rest being back EMF) \$\endgroup\$ – Brian Drummond Jul 29 at 13:48
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    \$\begingroup\$ @BrianDrummond It depends whether you've designed with PMDC motors, whether it's appropriate to play down efficiency. I will grant that if we call speed and torque constants first order ones, then efficiency is second order. However, we can freely choose the first by varying the number of turns, we have to pay serious money in quality to improve the second. When 50% is common for small motors, and the 1 kW ones I built into my fighting robot got only 75% at rated power (20% of stall torque), I wouldn't use the word 'only' for operating losses. \$\endgroup\$ – Neil_UK Jul 29 at 14:32
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    \$\begingroup\$ @Neil_UK Agree with all of that. What I was getting at is that what you lose in torque from a wider gap isn't pure inefficiency. It's partly inefficiency, partly increased speed (because lower Kt is higher Kv). The lower the efficiency in the first place, the more you lose this way. If you are using a motor in the 85% efficiency range or above, it becomes a bit less important. \$\endgroup\$ – Brian Drummond Jul 29 at 17:26
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Heat drives up the resistance of the windings, thus you loose torque. Excessive heat can weaken the permanent magnets, thus loss of torque. Air gaps of 10 mils are common. The issue with very tight 1 mil air gaps is when bearings wear out, the rotor has a slight wobble and if it hits the stator that is bad news.

To compensate ultra-hard bearings can be used, like the silicon nitride bearings used in the Space Shuttle, 1/10th the cost of synthetic sapphire bearing and lasting 5 times longer.

But this makes for a very expensive motor, and marketing often has more say than engineering. Bearings made of hard nickel-steel alloys would be more common.

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